The STK600 includes a RS232 hardware that can be used for communication between the target AVR microcontroller in the socket and a PC serial port. STK600 has a 9-pin DSUB connector that can be connected to a PC with a straight serial cable (not a null modem cable).
To use the RS232 interface, the AVR's UART pins must be connected to the appropriate pins on the "RS232 SPARE" pin header. Use a 2-wire cable to connect the AVR's RXD and TXD pins to the pin header. The "RS232 SPARE" pin header is found in the target header section, while the DSUB marked "RS232" is located on the other end of the card.
Optionally one can connect the RTS (Request To Send) and CTS (Clear To Send) signals to two free I/O ports. The RTS and CTS signals are used for flow control. The connection is shown below.
If the RTS and CTS lines are not controlled by the AVR, a jumper shorting the RTS and CTS pins on the "RS232 SPARE" header can resolve communication problems, if the PC side expects these handshake lines to be active.
Flow control is used to avoid data loss in transmission when one party is unavailable to receive data. When a DTE (such as a PC) wants to stop the data flow into it, it negates RTS. (read a negated "Request To Send" as "request NOT to send to me" (stop sending)). When the PC is ready for more bytes it asserts RTS and the flow of bytes to it can resume. Flow control signals are always sent in a direction opposite to the flow of bytes that is being controlled. DCE equipment (I.E. AVR) works the same way but sends the stop signal out the CTS pin (negated CTS: "(you are) NOT Cleared To Send").